Many consumer electronic devices are now being built with touch sensitive screens, for use with finger or stylus touch user inputs. These devices range from small screen devices such as mobile phones and car entertainment systems, to mid-size screen devices such as notebook computers, to large screen devices such as check-in stations at airports.
Most conventional touch screen systems are based on resistive or capacitive layers. Such systems are not versatile enough to offer an all encompassing solution, as they are not easily scalable.
Reference is made to FIG. 1, which is a prior art illustration of a conventional touch screen system. Such systems include an LCD display surface, a resistive or capacitive overlay that is placed over the LCD surface, and a controller integrated circuit (IC) that connects to the overlay and converts inputs from the overlay to meaningful signals. A host device (not shown), such as a computer, receives the signals from the controller IC, and a device driver or such other program interprets the signals to detect a touch-based input such as a key press or scroll movement.
Reference is made to FIG. 2, which is a prior art illustration of a conventional resistive touch screen. Shown in FIG. 2 are conductive and resistive layers 1 separated by thin spaces. When a pointer 2, such as a finger or a stylus, touches the screen, a contact is created between resistive layers, closing a switch. A controller 3 determines the current between layers to derive the position of the touch point.
Advantages of resistive touch screens are their low cost, low power consumption and stylus support.
A disadvantage of resistive touch screens is that as a result of the overlay, the screens are not fully transparent. Another disadvantage is that the screens require periodic re-calibration. Another disadvantage is that pressure is required for touch detection; i.e., a pointer that touches the screen without sufficient pressure goes undetected. As a consequence, resistive touch screens do not work well with fingers. Another disadvantage is that resistive touch screens are generally unreadable in direct sunlight. Another disadvantage is that resistive touch screens are sensitive to scratches. Yet another disadvantage is that resistive touch screens are unable to discern that two or more pointers are touching the screen simultaneously, referred to as “multi-touch”.
Reference is made to FIG. 3, which is a prior art illustration of a conventional surface capacitive touch screen. As shown in FIG. 3, two sides of a glass substrate 1 are coated with a uniform conductive indium in oxide (ITO) coating. In addition, a silicon dioxide hard coating is coated on the front side of one of the ITO coating layers. Electrodes 2 are attached at the four corners of the glass, for generating an electric current. A pointer 3, such as a finger or a stylus, touches the screen, and draws a small amount of current to the point of contact. A controller 4 then determines the location of the touch point based on the proportions of current passing through the four electrodes.
Advantages of surface capacitive touch screens are finger touch support and a durable surface.
A disadvantage of surface capacitive touch screens is that as a result of the overlay, the screens are not fully transparent. Another disadvantage is a limited temperature range for operation. Another disadvantage is a limited capture speed of pointer movements, due to the capacitive nature of the touch screens. Another disadvantage is that surface capacitive touch screens are susceptible to radio frequency (RF) interference and electromagnetic (EM) interference. Another disadvantage is that the accuracy of touch location determination depends on the capacitance. Another disadvantage is that surface capacitive touch screens cannot be used with gloves. Another disadvantage is that surface capacitive touch screens require periodic re-calibration. Another disadvantage is that surface capacitive touch screens require a large screen border. As a consequence, surface capacitive touch screens cannot be used with small screen devices. Yet another disadvantage is that surface capacitive touch screens are unable to discern a mufti-touch.
Reference is made to FIG. 4, which is a prior art illustration of a conventional projective capacitive touch screen. Shown in FIG. 4 are etched ITO layers 1 that form multiple horizontal (X-axis) and vertical (Y-axis) electrodes. AC signals 2 drive one axis and the response through the screen loops back via the other electrodes. Location of a pointer touching the screen is determined based on the signal level changes 3 between the horizontal and vertical electrodes.
Advantages of projective capacitive touch screens are finger mufti-touch detection and a durable surface.
A disadvantage of projective capacitive touch screens is that as a result of the overlay, the screens are not fully transparent. Another disadvantage is their high cost. Another disadvantage is a limited temperature range for operation. Another disadvantage is a limited capture speed, due to the capacitive nature of the touch screens. Another disadvantage is a limited screen size, typically less than 5″. Another disadvantage is that surface capacitive touch screens are susceptible to RF interference and EM interference. Yet another disadvantage is that the accuracy of touch location determination depends on the capacitance.
It will thus be appreciated that conventional touch screens are impractical for general use with small mobile devices and devices with large screens.